Whosoever observes Saum (fast) for one day for Allaah’s Cause ..

“Whosoever observes Saum (fast) for one day for Allaah’s Cause, Allaah will keep his face away from the (Hell) Fire for (a distance covered by a journey of) seventy years.”

[Saheeh Al-Bukhaari, Hadeeth No. 93, Vol. 4]

Source : The Book Of As-Siyaam (the fasting) – Chapter 31- The superiority of observing Saum (fast) in Allaah’s Cause by the one who has the power to do it without getting hurt (by doing so) or overlooking his duties.
Al-Lulu Wal-Marjan,Compiled by : Fuwad Abdul-Baqi,Translated by : Dr. Muhammad Muhsin Khan